The present invention relates to an image sensor, and more particularly, to an image sensor which senses light reflected by an original sheet by utilizing photoelectric converters and generating image signals, a method of assembling the image sensor, and an image processing apparatus using the image sensor.
Conventionally, a contact-type image sensor is often used as an original-sheet reading apparatus employed by a facsimile apparatus, copying machine, scanner or the like. The contact-type image sensor projects a read image onto each sensing element on one-to-one basis by using a sensor array, and obtains an image having a size identical to the read original sheet.
The conventional contact-type image sensor comprises: a sensor chip where a plurality of elements for performing photoelectric conversion are arrayed; a sensor substrate on which the sensor chip and protection film which protects the sensor chip are mounted; illumination means for illuminating an original sheet; a rod lens array for forming an image of the original sheet onto the elements of the sensor chip; and a cover glass serving as an original-reading surface, and the foregoing components are fixed on a frame, serving as a base, by adhering, inserting, press-fitting, caulking or the like.
An example of the image sensor of this type is described with reference to the cross-section in FIG. 8 and the exploded perspective view in FIG. 9. Referring to FIGS. 8 and 9, the conventional image sensor consists of a sensor chip 1, sensor substrate 2 for mounting the sensor chip 1, rod lens array 3, illumination means 8, cover glass 5 and frame 6 serving as a base for maintaining the positions of the above components. Note that as shown in FIG. 9, the illumination means 8 comprises an LED unit 7 serving as a light source and a light guide 4.
In the illumination means 8, light emitted by the LED unit 7 and propagated inside the light guide 4 in the longitudinal direction is reflected and diffused by a reflection/diffusion portion 4a of the light guide 4, and irradiated externally, illuminating the original sheet 10. An image of the irradiated original sheet 10 is formed by the rod lens array 3, on the photoelectric converters arrayed on the sensor chip 1, and photoelectrically converted.
The image signals outputted by the sensor chip 1 are transmitted to an external circuit via a connector (not shown) provided on the sensor substrate 2.
In recent years, as the contact-type image sensor of this type is built into portable apparatuses or incorporated into printers or keyboards or the like, there is increasing demands for low electrical consumption and a reduced size and weight of the image sensor.
However, in the above-described conventional contact-type image sensor, the following problems arise in order to reduce the electrical consumption, size and weight of the image sensor.
First, to reduce electrical consumption, it is necessary to decrease the amount of electricity consumed by the illumination portion. For instance, in a case where linear illumination is provided by a smaller amount of light by using the light guide 4, the light-convergence width must be reduced to efficiently use the amount of illumination light. As a result, there are more possibilities of deviation being generated between the illumination position and original-reading position, thus giving negative influence to uniformity of sensor outputs.
Furthermore, for reducing the size and weight of the image sensor, a shorter distance (conjugate length Tc) between an original and image sensing surface of the rod lens array 3 is provided. However, by having a shorter conjugation length Tc, the amount of light provided tends to be low due to deviation in optical axes of the rod lens array 3 and sensor chip 1. As a result, non-uniformity of sensor outputs become conspicuous.
Moreover, as the size and weight of the apparatus is reduced, the rigidity of each component declines; as a result, positioning deviation is more likely to be generated due to deformation such as warp or the like of each component. Consequently, the above-described problems become more serious.